History and Structure of the Nuclear Atom

The Atom

• smallest particle of an element that retains all properties of the element.

I. Early Models of the AtomA. Democritus (460 B.C. – 370 B.C) • first to suggest the

existence of atoms• believed atoms were

indivisible and indestructible

B. Dalton

• Atoms of the same element are identical

• Each element is unique• Atoms combine in fixed ratios

to form compounds

Name of theoryBillard Ball

Model

C. J.J. Thomson• Discovered electron• Suggested atom looked like plum pudding (or a

blueberry muffin or chocolate chip cookie) with electrons evenly distributed throughout positive sphere

Name of theoryPlum Pudding

Model

D. Rutherford• Conducted gold foil experiment

What did Rutherford expect?

Gold Foil Experiment Animation and Discussion

What did Rutherford actually see?

D. Rutherford

• Conducted gold foil experiment• Proposed atom is mostly empty space• Concluded all positive charge and mass is

concentrated in small region called nucleus

Name of theoryNuclear atom

D. Rutherford• All the positive charge and all the mass of the

atoms was concentrated in a small region called the nucleus

• “This is almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you.” – Rutherford, 1938

E. Bohr• depicts the atom as a small, positively

charged nucleus • surrounded by electrons that travel in

circular orbits around the nucleus

Name of theoryPlanetary Model

F. Schrödinger• model allowed the electron to occupy

three-dimensional space like an electron “cloud”

• Used a mathematical equation to show a model of electrons as waves

Name of theoryQuantum Mechanical

Model or Electron Cloud Model

A. Size• Teeny tiny• observable with instruments

such as a scanning tunneling microscope.

II. More about the Atom

B. Parts (called Subatomic Particles)1. Protons: p+

– charge = positive +1– mass =1

2. Electrons: e-

– charge = negative -1– mass ~0

3. Neutrons: no

- charge = zero - mass = 1

C. Location of Parts• Nucleus: center of atom– contains protons and neutrons– has a positive charge– contains almost ALL of the mass of the atom

• Outside nucleus: – Contains electrons– Has a negative charge– Occupies almost ALL of the volume of the atom

D. Atomic Charge• Atoms are NEUTRAL• # of protons = # of electrons (ALWAYS if you’re talking about an ATOM)

Fun Facts

• The volume of a hydrogen nucleus is a trillion times smaller than the volume of a hydrogen atom, yet the nucleus contains most of the mass.

• If the nucleus (proton) of a hydrogen atom were as large as the width of a human thumb, the electron would be on the average about one kilometer away in a great expanse of empty space.